1. Field of the Invention
The present invention relates to method and apparatus for adjusting an apparatus using a semiconductor laser used in a recording apparatus such as an electrographic laser printer or a digital copying machine.
More particularly, the present invention relates to a method for adjusting a firing time when such a light source is fired for each pixel and an apparatus used therefor and a recording apparatus suitably adjusted by the method. Further, specifically, the present invention is preferably implemented in a recording apparatus in which a light source is fired for a time in accordance with each pixel density level.
2. Related Background Art
A color page printer using an electrographic system has been attracting an interest because of its high quality, multiple of tones and high printing speed. For a full color laser page printer, a process of scanning a laser beam along a main scan line to conduct first development and then transferring a resulting latent image onto a record sheet on a transfer carrier is repeated four times to conduct multi-color printing. The four processes correspond to recording processes for Y (yellow), M (magenta), C (cyan) and K (black).
In such a color page printer, because of present electrographic process technology and toner diameter, different processes are used for a natural image (halftone image) such as a photograph and a character image (binary image) in order to enhance the print qualities thereof. Namely, for the character image, the printing is made at 400 to 600 dpi(dots per inch) putting a priority on a resolution and for the natural image, the printing is made at 400 to 600/N lines with N pixels in a set to put a priority to the tonality.
Recently, a multi-value printout printer in which halftone image data from a host computer is received as non-binary (for example, dither system) multi-value image data by 8 bits per pixel, and a light source is modulated by a pulse width modulation signal for each pixel has been put into practice.
As a light source unit for such a color printer, a semiconductor laser unit having a collimator lens for converting a diverging light beam from a semiconductor laser to a collimated light beam is frequently used. A laser exit light from the semiconductor laser forms an electrostatic latent image on a photo-conductor through scanning means and focusing means and it is transferred to a recording sheet to form an image printout. Such a semiconductor laser unit is usually mounted in a scanner of the color page printer.
In order to present constant density printing in a print operation by the printer using the above pulse width modulation method, it is necessary to continuously control an output power of the semiconductor laser to a standard power setting. To this end, in the prior art, a feedback control loop as shown in FIG. 15 is formed to conduct light intensity control (APC). Namely, a back beam which is proportional to a light intensity of a semiconductor laser 137' is sensed by a monitoring photodiode 138' and an output value S138 thereof is compared with a reference value Vt by a signal comparator 130' to adjust a drive current of the semiconductor laser 137' such that both values match.
However, in a mutual relation between the output power of the semiconductor laser 137' and the light intensity detection value S138 of the monitoring photodiode 138', a variation of a scanning optical system including the semiconductor laser unit has an noticable amount due to a variation of a divergence angle of the semiconductor laser 137', a variation of a relative positional balance and sensitivities of the monitoring photodiode S138' and the semiconductor 137', a variation of a scanning system efficiency such as a collimator lens, a variation of linearity of a light intensity of the semiconductor laser 137' to input drive data and a variation of a filtering characteristic of the light intensity due to mismatching of an operational specific resistance in a chip of the semiconductor laser 137' and a pattern impedance of a drive circuit.
When such variations are included, a predetermined laser power for the input data from the host computer is not produced on the printer. As a result a print density varies from semiconductor laser unit to unit and sufficient tonality to represent the pulse width modulated data is not attained.
Thus, an output power of a front beam of the semiconductor laser 137' is measured by an optical power meter at a position corresponding to a mount position of a photo-conductor which is a final illumination position of the laser beam to adjust a drive current of the laser drive circuit 134', and an amplification gain of the monitor output used in the APC operation of the recording apparatus is adjusted such that the laser power at the photo-conductor mount position matches to the standard power setting, and a correction is made such that the monitor detection value S138 matches to the reference value Vt.
For the pulse width modulation, like the APC operation, the output power of the front beam of the semiconductor laser 137' is measured by the optical power meter at the photo-conductor mount position which is the final illumination position of the laser beam, and the signal pulse width is adjusted such that average laser powers by modulation signals of a minimum pulse width and a maximum pulse width generated from a pulse width modulation generator (not shown) match to reference values Vp(min) and Vp(max).
In the above color page printer, an optimum resolution (hereinafter referred to as the number of lines) is different between the character image and the natural image, and for the pulse width adjustment, a minimum pulse width and a maximum pulse width are adjusted for each of different numbers of lines.
FIGS. 16 and 17 illustrate a relation between the pulse width adjustment and the photo-conductor surface. A pulse waveform shown by (1) in FIG. 16 is a theoretical laser emission response waveform having a target value at a laser beam energy and (2) shows a relation between a laser output light intensity and the pulse width adjustment when a light intensity energy to a minimum pulse width modulation signal (data from the host computer is `OOh`) is under. A waveform (3) shows a relation between the laser output light intensity and the pulse width adjustment when the light intensity energy is over. FIG. 17 is a graphic representation of the mutual relation of (1)-(3) of FIG. 16.
A PWM pixel modulation method for a laser beam printer (LBP) has been known in the art, in which a laser beam radiation time is controlled for each pixel to obtain a light amount correlated to the printing density (deposited toner amount) suitable for a highly fine (high gradation) video image.
FIG. 18 illustrates such a pixel modulation. A video clock (FIG. 19A) representing a pixel unit and synchronizing with a beam detect (BD) pulse indicating a horizontal reference position of a printing sheet, is inputted to an input terminal 40. The video clock signal is converted into a ramp wave (FIG. 19D) synchronously with the video clock signal, and the ramp wave signal is supplied to a comparator 41. Pixel data (FIG. 19B) of eight bits, for example, is inputted to an input terminal 45, the pixel data being used for determining the printing density of each pixel. The input pixel data is latched (FIG. 19C) by a latch 46 in response to the video clock. An output of the latch 46 is converted into an analog voltage (FIG. 19D) by a D/A converter 42, the analog voltage being supplied to the comparator 41. As shown in FIG. 19D, the comparator 41 compares the input ramp wave signal with the pixel analog voltage to output a laser drive pulse (FIG. 19E) pulse width modulated in accordance with the density of pixel data.
A laser beam is radiated, for example, while the laser drive pulse takes an H level. Therefore, the pixel data DN+2 corresponds to a `deep pixel`, and the pixel data DN corresponds to a `light shaped pixel`. The printing density is very sensitive to a pulse width (radiation time). For a high image quality, it is therefore necessary not only to be able to change a peak level value and a DC offset value of the ramp wave in accordance with the environmental conditions, but also to make the ramp wave stable.
In the ramp wave generator circuit 43 shown in FIG. 18, the video clock is shaped by a buffer 44 to eliminate noises such as ringing, and transformed into a ramp wave by a time constant circuit having a time constant T=R31*C13 which is larger than a clock period.
The level of the ramp wave can be set by the resistance value of R31, and the DC offset can be set by VR1 with a sufficiently large capacitance of C14. In order to ensure the linearity of the ramp wave slope, it is necessary to set the time constant T about three times the video clock period.
In the method for finely adjusting the semiconductor laser beam power by varying the pulse width of the pulse width modulation signal as the potentiometers VR1 and R31 are rotated for the multi-tone reproduction such that an integrated light energy reached a predetermined level, the following problems are encountered:
(1) For the pulse width modulator, when the ramp wave from the ramp wave generation means 43 and the DA-converted value of the data from the host computer are compared by the comparator 41 to generate the pulse width modulation signal, the data are set to "OOh" (h represents hexadecimal) corresponding to a lowest density and "FFh" corresponding to a highest density and the potentiometers VR1 and R31 are adjusted to adjust the maximum pulse width and the minimum pulse width.
In this case, the two adjustment units are formed by potentiometers and optimum control is required for each of the character image and the natural image. Accordingly, a total of four potentiometer adjustments are required in connection with the pulse width modulation.
Further, in the adjustment by the potentiometer, it is necessary to lock the potentiometer by paint so that the adjusted position is not changed by mechanical vibration after the adjustment.
(2) It is difficult to automate the power adjustment and the adjustment takes time because there are four adjustments in connection with the pulse width modulation. In the APC adjustment, the adjustment is made while the laser is DC driven, and in the pulse width adjustment, the adjustment is made while the laser is AC driven by the minimum and maximum pulse width signals so that the light integration energy detected by the optical power meter is small and delicate and sensitive to jitter. Thus, the adjustment takes time. Further, when VR1 is adjusted while the data "OOh" is outputted to determine the minimum pulse width and then R31 is adjusted while the data "FFh" is outputted, the minimum pulse width varies, that is, the minimum pulse width and the maximum pulse width cannot be adjusted independently. As a result, the adjustment is not readily attained.
(3) The variable range of the potentiometer may be reduced to facilitate the adjustment of the potentiometer, but when laser chips of different manufacturers are used in common to reduce a cost of the pulse width adjustment unit, the laser characteristic varies from manufacturer to manufacturer and even the laser chip of the same manufacturers vary from lot to lot. In addition, when a variation of the scanning optical system extending to the photo-conductor is taken into consideration, the variable range of the potentiometer cannot be significantly reduced.
Thus, a multi-rotation potentiometer or a pair of potentiometers for coarse adjustment and fine adjustment may be used for each adjustment position (in the above example, eight potentiometers are required) but this results in the increase of a cost of the apparatus.
(4) When a laser light emission time T is sufficiently long relative to a rise time tr and a fall time tf, the light transient response of the semiconductor laser (ringing such as overshoot and undershoot) raises no problem because an error of the light energy produced at the photo-conductor mount position is negligible.
However, for the pulse width modulation signal, particularly for the minimum pulse width data, the laser light emission time T is comparatively short compared with the rise time tr and the fall time tf and the error of the light energy produced at the photo-conductor mount position is not negligible.
In this case, the variation may be absorbed to some extent as shown in FIG. 16 by varying the laser light emission time by the pulse width modulation to adjust the light energy but it is not sufficient. FIG. 20 shows a relation between the pulse width modulation signal, the semiconductor laser response waveform and the light energy on the photo-conductor.